(1) Field of the Invention
The present invention relates to luminance and chrominance signal separating apparatus for separating a luminance signal and a chrominance signal from an NTSC composite color television signal.
(2) Description of the Prior Art
Recently, progress of digital signal processing technique accompanies that an adaptive type luminance and chrominance signal separating apparatus which selects a plurality of filters according to exsistence of correlation between an arbitrary reference signal and the signals at the vicinity of the position of the reference signal in a horizontal or vertical direction on the screen, as a luminance and chrominance signal separating apparatus which separates a luminance signal and a chrominance signal from a composite color television signal.
FIG. 1 is a block diagram of a luminance and chrominance signal separating apparatus in accordance with the prior art.
A bandpass filter 501 extracts a chrominance signal component from a composite color television signal (hereafter, called VBS signal, which means video burst signal). Each one of 1H delay devices 502, 503 and 504 delays a signal by 1H period, where 1H means one horizontal period. In digital circuits, memory devices are usually used as delay devices. A comb filter 505 is supplied with the output signal of the bandpass filter 501 and the output signal of the delay device 502 which is delayed from the output signal of the bandpass filter 501 by 1H period and outputs a chrominance signal. A comb filter 506 is supplied with the output signal of the delay device 502 which is delayed from the output of the bandpass filter 501 by 1H period and the output signal of the delay device 503 which is delayed from the output of the bandpass filter 501 by 2H periods and outputs a chrominance signal.
An adder 507 adds the output signal of the bandpass filter 501 and the output signal of the delay device 502 which is delayed from the output signal of the bandpass filter 501 by 1H period. An adder 508 adds the output signal of the delay device 502 which is delayed from the output signal of the bandpass filter 501 by 1H period and the output signal of the delay device 503 which is delayed from the output signal of the bandpass filter 501 by 2H periods.
Absolute value circuits 509 and 510 calculate absolute values of the output signals of the adders 508 and 507, respectively. Low pass filters 511 and 512 filter the output signals of the absolute value circuits 509 and 510, respectively and output their average values. A comparator 513 compares the output signals of the low pass filters 511 and 512 and judges which correlation is stronger. A smaller average value means stronger correlation. A converting circuit 514 converts the output signal of the comparator 513 into a mixing coefficient of the comb filters 505 and 506. A band pass filter 516 limits the bandwidth of the chrominance signal which is an output of the mixing circuit 515. A subtracter 517 subtracts the output of the bandpass filter 516 which is a final chrominance signal from a signal which is delayed from the input VBS signal by 1H period.
A chrominance signal is extracted from the input VBS signal at the bandpass filter 501. The extacted chrominance signal component is applied to the delay devices 502 and 503 in order and one input signal to the delay device 502 and the two output signals from the deley devices 502 and 503 are applied to the two comb filters 505 and 506. Because each comb filter 505 and 506 is supplied with two signals which have an 1H period time difference, the output of each comb filter becomes a chrominance signal.
While, two pairs of signals, each pair of which has an 1H period time difference, are added at the adders 508 and 507, the sums are outputted to the following absolute value circuits 509 and 510, absolute value signals are filtered at the low pass filters 511 and 512 and average values of the absolute values are outputted from the low pass filters 511 and 512. These values are vertical correlation values. The correlation values are compared at the comparator 513. The smaller the value is, the stronger the vertical correlation is. The comparison result is converted to mixing coefficients for the comb filters 505 and 506. The mixing circuit 515 outputs an optimum chrominance signal by weighting the output of the comb filters 505 and 506 according to the mixing coefficients and mixing the two weighted signals. The output chrominance signal from the mixing circuit 515 is limited with its frequency band at the bandpass filter 516. A perfect chrominance signal is outputted from the bandpass filter 516 by limiting the frequency band of the output signal of the mixing circuit 515. While, a perfect luminance signal is obtained by subtracting the perfect chrominance signal from the bandpass filter 516 from a signal delayed by 1H period from the input VBS signal at the subtracter 517.
However, in the above configuration, because the bandpass filter 516 which limits the bandwidth of the chrominance signal has a fixed frequency characteristic, if the bandwidth is wide, the color transient characteristic is good but cross color interference is remarkable when there is no vertical correlation and if the bandwidth is narrow, the color transient characteristic is bad. This is a problem.